The present invention relates to a pulse oximetry which measures continuously arterial oxygen saturation (SaO2) in a non-invasive manner by using a blood volume variation of the tissue arterial blood caused by pulsation, and also to a pulse oximeter which performs such a pulse oximetry.
Today, in a related-art technique called a pulse oximetry, in the case where SaO2 is to be obtained, the following procedure is usually adopted.
(1) Tissue transmitted or reflected light is continuously measured at a plurality of wavelengths.
(2) The peak and bottom of pulsation of the measured tissue transmitted or reflected light are determined, and the transmitted or reflected light beams at the peak and bottom are indicated by L+ΔL and L, respectively.
(3) ΔA≡log [(L+ΔL)/L]≈ΔL/L is obtained.
(4) Φij≡ΔAi/ΔAj is obtained.
(5) Since Φij corresponds on a substantially one-to-one base to SaO2, Φij is converted to SaO2.
Many apparatuses which are currently commercially available, and which measure SaO2 employ two wavelengths, and, when above-described Φij is to be converted to SaO2, use a conversion table. In the case of a two-wavelength apparatus, the use of a conversion table is not particularly problematic. In the case where a larger number of wavelengths are used in order to improve the measurement accuracy, however, the conversion must be performed by using a calculation expression which is obtained theoretically and experimentally.
For example, as a related-art apparatus which measures continuously SaO2 in a non-invasive manner by using a volume variation of arterial blood caused by pulsation, there is a five-wavelength pulse oximeter which irradiates living tissue with five light beams of different wavelengths (see JP-A-2005-95606).
The pulse oximeter disclosed in JP-A-2005-95606 includes: a light emitting portion which irradiates living tissue with five light beams of different wavelengths; a light receiving portion which receives the light beams that are emitted from the light emitting portion, and that are transmitted through or reflected from the living tissue, and which converts the light beams to electric signals; an optical-density-variation calculating portion which obtains optical density variations for the living tissue on the basis of variations of the transmitted or reflected light beams of different wavelengths and output from the light receiving portion; an optical-density-variation-ratio calculating portion which obtains at least four of mutual ratios of five optical density variations obtained in the optical-density-variation calculating portion; and an oxygen saturation calculating portion which, based on the optical-density-variation ratios obtained in the optical-density-variation-ratio calculating portion, calculates oxygen saturation in blood while using four unknowns of the SaO2, the venous oxygen saturation, a ratio of variations of arterial blood and venous blood, and a tissue term, and obtains oxygen saturation of arterial blood while eliminating artifacts of variations of venous blood and the tissue.
According to the thus configured pulse oximeter disclosed in JP-A-2005-95606, in the case where venous blood is pulsated by any reason, an artifact of the pulsation can be surely eliminated, and SaO2 can be accurately measured without producing a time delay and smoothing. In the case where the pulse wave is so small that a pulse oximetry is impossible, body motion is intentionally applied, thereby enabling SaO2 contained in this case to be obtained. The pulse oximeter has another advantage that also venous oxygen saturation can be simultaneously measured.
A longstanding problem of a pulse oximetry is that transmitted or reflected light is disturbed by mechanical disturbances such as body motion. Namely, disturbances of transmitted or reflected light cause adequate detection of peaks and bottoms of a measured pulsative waveform to be hardly performed.
A related-art technique which has been proposed or adopted as a countermeasure against these problems is a statistical technique in which the correct value of SaO2 is estimated from preceding and subsequent data. However, the technique has the following problems.
(1) Since a long time delay is produced, detection of, for example, a start of reduction of SaO2 is delayed.
(2) Changes of SaO2 are smoothed. When SaO2 is largely reduced, for example, the degree of the reduction cannot be known.
In the related-art pulse oximetry technique, it will be further expected that a change in SaO2 of a patient is quickly detected, so that the change is early coped with. In order to utilize the original feature of the pulse oximetry technique, the above-discussed problems must be solved.
Furthermore, it has been found that, in the related-art pulse oximetry technique which is based on determination of peaks and bottoms of the pulsative waveform of measured transmitted or reflected light cannot obtain a satisfactory measurement result when the body a patient is vigorously moved.
From this viewpoint, the following related-art technique has been proposed, a time-segmented pulse oximetry and pulse oximeter in which, with respect to the pulsative waveform of measured transmitted or reflected light, the whole of a signal of the measured transmitted or reflected light is used, whereby SaO2 can be adequately measured (see JP-A-2007-90047).
In the related-art technique disclosed in JP-A-2007-90047, the whole of time series data of the transmitted or reflected light is used, so that determination of peaks and bottoms of the measured waveform is not necessary. Namely, the related-art technique disclosed in JP-A-2007-90047 is characterized in that light emitting elements irradiate living tissue with a plurality of light beams of different wavelengths, light receiving elements receive the light beams transmitted through or reflected from the living tissue, and converts the respective light beams to electric signals, time series data of the electric signals obtained by the light receiving elements are time-segmented, with respect to the segmented time series data, slope values of regression lines between two different wavelengths are calculated, the calculated slope values are converted to SaO2, respectively, and then smoothing is performed, thereby obtaining SaO2 from which an artifact of body motion is eliminated.
According to the related-art technique disclosed in JP-A-2007-90047, with respect to the pulsative waveform of measured transmitted or reflected light, therefore, determination of peaks and bottoms of the measured waveform is not performed, and the whole of time series data of the transmitted or reflected light is used, whereby an artifact of body motion is eliminated, contribution to improvement of the measurement accuracy of SaO2 is obtained, and measurement flexibility of a measurement portion can be enhanced. Even with these two related-art techniques, an artifact of the body motion can not be eliminated sufficiently.